Method of thermal spray coating fiber-reinforced composite materials

ABSTRACT

A method of coating a fiber-reinforced composite material includes providing a fiber-reinforced composite workpiece, treating a surface of the workpiece to remove at least a portion of a polymer matrix and to expose fibers to a treated surface of the workpiece, and coating the treated surface of the workpiece using a thermal spray coating process. The treatment can include laser ablating or applying a peel ply process. The method yields a coated fiber-reinforced composite workpiece with the coating being bonded directly to the fibers via diffusion of the coating into the fibers, the formation of an intermediate compound at an interface of the coating and the fibers, or both.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/490,416 filed on Apr. 26, 2017, the entire content of which is hereby incorporated by reference herein.

BACKGROUND

The present invention relates to fiber-reinforced composites and to methods of applying functional coatings to fiber-reinforced composites.

Fiber reinforced composites (FRC's) follow the designs seen in nature, where strong fibrous materials are held together with a matrix to form strong lightweight structures. One common example is the limbs of trees where the span of the limb can be an order of magnitude more than the diameter of the limb and still withstand high loads from foliage, fruit, and winds. Natural composites are self-healing and damage tolerant, unlike the manmade composites which require some form of surface treatment to function in harsh environments.

Manmade, fiber reinforced composites vary widely in construction. The most common fibers are carbon, glass, and natural or engineered organic fibers such as cotton or Kevlar® respectively. Fibers can be randomly distributed in a matrix or woven into various cloth-like patterns in 2D and 3D weaves. Matrices which hold the fibers together also vary widely according to the intended use. Matrices include such materials as metals, metal alloys, ceramics, carbon, epoxy, phenolic, PTFE, Nylon®, and many more. Carbon-carbon (C—C) composites are another example, used more for high temperature applications such as aircraft disc brakes.

Thin film protective coating techniques such as physical vapor deposition (PVD), paints, and polymers will adhere to most FRC's. For many applications the 25 μm (0.001 in.) thickness limitation of PVD coatings is restrictive. Paints and polymer coatings are limited to applications for aesthetics and minor abrasion or erosion resistance. Chemical vapor deposition (CVD) can be used with C—C composites, where the high temperatures of deposition do not affect the matrix. With CVD, coating thicknesses of 250 μm (0.010 in.) are possible.

Low melting point materials have been applied to FRC's using various sub types of the thermal spray processes. The processes are usually electric arc spray (EAS), wire flame spray, or atmospheric plasma-arc spray (APS); the materials usually include tin, zinc, and aluminum and their alloys, and in some instances silicon bronze. The nominal melting points of these lower temperature materials are: tin 232 C, zinc 420 C, aluminum 660 C, and silicon-bronze 1000 C. There are variants, but these represent the majority of applications. In most applications, these coatings are used for EMI/RFI shielding or as bond coats for other coatings, paints, or adhesives.

SUMMARY

Thermal spray techniques are well suited for use where thick (e.g., >25 μm), functional coatings are required and where the matrices cannot tolerate the high temperatures of PVD and CVD. Thermal spray coatings are usually inorganic and can be applied in incremental thicknesses from a few micrometers to single digit millimeters. Thermal spray coatings are often used for EMI/RFI shielding, X-ray shielding, heavy erosion, thermal barriers, liquid metal attack, oxidation protection, etc. It is not difficult to apply thermal spray coatings to metal matrix composites (MMC's), ceramic matrix composites (CMC's) and carbon/carbon composites. The difficulty lies in applying thermal spray coatings to FRC's that are carbon or glass reinforced polymers (CFRP or GFRP), the matrices of which are incompatible with high temperature coating materials desired for use in industry and defense. When higher melting point materials, i.e., materials with melting points greater than 1000 C, are sprayed onto FRC's, the molten droplets damage (e.g., burn) the polymer surface preventing adhesion.

This invention deals specifically with applying high-melting-point materials to FRC's that are carbon- or glass-fiber reinforced polymers (CFRP or GFRP). In order to create adhesion to CFRP's and GFRP's with high melting point materials, the first surface matrix material is at least partially removed. In one embodiment of this invention, laser ablation is used to first evaporate the polymer matrices from the line of sight surface, exposing the glass or carbon fibers. With the matrix removed and the reinforcing fibers exposed, coatings can be applied using thermal spray techniques or other coating techniques that apply high-melting-point materials to the FRC's. Laser ablation results in a complex, high surface area region or surface of exposed fibers to which the sprayed material is applied.

Another alternative process to create a bondable surface is termed peel ply. Peel ply is a woven synthetic removable fabric that must be placed as the last layer in direct contact with a FRC's polymer matrix, and cured to the surface to be bonded. Those of skill in the art will be familiar with available peel ply products and their usage. After cure and prior to the thermal spray process, the peel ply is removed, leaving a woven pattern on the bonding surface. This woven pattern may be formed in the polymer matrix itself, or in some instances, may expose woven fibers to a limited extent to partially create the woven pattern (along with polymer matrix material). In some embodiments, further polymer matrix can be removed from a peel ply treated surface by adding a post mechanical treatment such as polishing, sanding, or grit blasting. However, the additional mechanical treatment can, in some instances, damage exposed reinforcing fibers. Peel ply is a manual process best suited for primitive geometries.

Once the polymer matrix is at least partly removed, by either technique, preferred bonding mechanisms for thermal spray can be engaged to securely adhere the coating to the exposed fibers. Adhesion to the exposed fibers occurs as with any thermal spray coating by diffusion of elements of the coating material with the fibers, formation of an intermediate compound at the interface of the coating and fiber, Van der Waals forces, or various combinations of one of more of these mechanisms.

In one embodiment, the invention provides a method of coating a fiber-reinforced composite material. The method includes providing a fiber-reinforced composite workpiece, laser ablating a surface of the workpiece to remove at least a portion of a polymer matrix and to expose fibers to a treated surface of the workpiece, and coating the treated surface of the workpiece using a thermal spray coating process.

In another embodiment, the invention provides a method of coating a fiber-reinforced composite material. The method includes providing a fiber-reinforced composite workpiece, treating a surface of the workpiece with peel ply, followed by polishing, sanding, or grit blasting, to create a treated surface for bonding, and coating the treated surface of the workpiece using a thermal spray coating process.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate copper on glass ceramic diffusion.

FIGS. 2A and 2B are SEM images of aluminum on borosilicate glass showing formation of aluminum silicate.

FIG. 3 is a free-standing shape produced by spraying tungsten powder onto a polished copper preform, demonstrating the tenacity of Van der Waals forces for adhesion.

FIG. 4 illustrates a top surface of a glass epoxy coupon after peel ply treatment.

FIG. 5 illustrates an alumina coating on peel ply treated quartz substrate.

FIG. 6 illustrates a carbon fiber coupon and a fiberglass coupon prior to applying a laser ablation process.

FIG. 7 illustrates top surfaces of the coupons of FIG. 6 after laser ablation.

FIG. 8 illustrates a SEM micrograph of the laser ablated carbon fiber coupon of ablation parameter A.

FIG. 9 illustrates a SEM micrograph of the laser ablated carbon fiber coupon of ablation parameter E.

FIG. 10 illustrates a SEM micrograph of the laser ablated carbon fiber coupon of ablation parameter F.

FIG. 11 illustrates a SEM micrograph of the laser ablated fiberglass coupon of ablation parameter A.

FIG. 12 illustrates a SEM micrograph of the laser ablated fiberglass coupon of ablation parameter C.

FIG. 13 illustrates a SEM micrograph of the laser ablated fiberglass coupon of ablation parameter D.

FIGS. 14A and 14B are illustrations of alumina on glass epoxy diffusion.

FIGS. 15A and 15B are illustrations of tungsten on carbon fiber, showing formation of tungsten carbide at the interface.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

Thermal Spray is a generic term for a group of coating processes used to apply metallic or non-metallic coatings. These processes are grouped into four major categories: plasma spray (which can include plasma-arc spray, suspension plasma spray (SPS), and solution precursor plasma spray (SPPS)), flame spray, electric-arc spray, and cold spray. These energy sources are used to heat and/or accelerate the coating material (in powder, wire, slurry, metalorganic, or rod form) to a molten or semi-molten state. The resultant particles are accelerated and propelled toward a prepared surface by either process gases or atomization jets. Upon impact, a bond forms with the surface with subsequent particles causing thickness buildup. Adhesion mechanisms include Van der Waals forces, diffusion, and formation of an intermediate compound or alloy. There is a persistent myth in the thermal spray industry that coating/substrate bonds are mechanical. FIGS. 1, 2, and 3 show examples of the three bonding mechanisms. Specifically, FIGS. 1a and 1B illustrate an example of copper on glass ceramic diffusion. FIGS. 2A and 2B illustrate an example of aluminum on borosilicate glass, showing the formation of aluminum silicate. FIG. 3 illustrates an example of a free-standing shape produced by spraying tungsten powder onto a polished copper preform. Copper and tungsten are immiscible, therefore, in this application, adhesion relies solely on Vander Waals forces. Part removal was accomplished by heating and using differential expansion to separate the tungsten form from the copper preform.

Existing surface preparation for thermal spray coatings on FRC's follows this general procedure: cleaning the surface with a degreaser (usually alcohol), grit blast lightly (120 to 240 grit media) to matte the surface (a visual dulling), and rinse again with alcohol to remove grit blasting debris. In some instances, plasma (corona) treatment of the surface is use to enhance bonding. The prepared surface is then free of oils and dust and displays a matte finish. However, with FRC's, the first surface, as seen by the sprayed droplets, still presents the polymer matrix which cannot tolerate high temperature materials or spray stream particulate with a high thermal mass.

Therefore, according to the present invention, the surface of the FRC to which the thermal spray coating is to be applied undergoes a different surface preparation. In order to create a surface to be bonded with high-melting-point materials (e.g., greater than 1000 degrees C.), in a first embodiment of the invention, peel ply is used first to treat the top surface of polymer matrix. Peel ply is a removable fabric that is cured to the top surface of the coupon polymer composite. Different types of peel ply fabrics (such as nylon, polyester, epoxy pre-impregnated polyester, and etc.) and deposition techniques can be used to create variable surface structures for bonding. After being cured and prior to the thermal spray process, the peel ply is removed, leaving a woven pattern behind. While it was initially thought that this woven pattern was that of exposed fibers, it has been found that the peel ply process did not actually remove enough of the polymer matrix to expose the fibers. Optionally, further polymer matrix can be removed from a peel ply treated surface by adding a post mechanical treatment such as polishing, sanding, or grit blasting. However, the additional mechanical treatment can damage exposed reinforcing fibers. Once the desired treated surface is obtained (with or without the optional post mechanical treatment), the coating can be applied using thermal spray coating techniques or other techniques that apply high-melting-point materials to the peel ply treated surface of the FRC's. Peel ply is a manual process best suited for primitive geometries. FIG. 4 illustrates a peel ply treated surface of a glass epoxy coupon. FIG. 5 illustrates an alumina coating on a peel ply treated quartz substrate.

The thermal spray coating of peel ply treated FRC surfaces has yielded acceptable adhesion of thermal spray coatings due to the creation of the woven pattern on the treated surface, which is still largely the polymer matrix without exposed fibers. The peel ply treatment alone has not succeeded in completely removing the polymer matrix to expose the glass or carbon fibers. If fiber exposure is desired, the additional mechanical treatments can be practiced after the peel ply fabric removal. However, care must be taken to minimize damage to the reinforcing fibers.

Another technique to create adhesion to FRC's is laser ablation. The inventors have found laser ablation to provide for better adhesion than that obtainable with the peel ply process described above. In this second embodiment of the invention, laser ablation is used first to evaporate the polymer matrices from the line of sight surface, exposing the glass or carbon fibers. Then the coating can be applied using thermal spray coating techniques or other techniques that apply high-melting-point materials directly to exposed fibers of the FRC's. Of course, the inventive techniques of laser ablation and peel ply treatments can also be used when applying lower-melting-point coatings (e.g., less than or equal to 1000 C) to FRC's, however, the inventive treatments are believed to provide the first available techniques for providing a way to coat FRC's with high-melting-point materials while avoiding the burning or degradation of the matrix surface.

In one embodiment, a 355 nm UV laser in pulse mode was used to perform the laser ablation. Of course, those skilled in the art will understand that other types of lasers and different wavelengths might also be used depending on the matrix material to be removed. Furthermore, the various parameters that can be used during the laser ablation process (e.g., power level, speed, standoff distance, translations, passes, index spacing, pitch/increment, beam shape, etc.) can be varied depending upon the matrix material to determine the optimal ablation process for any given FRC material. Every manufacturer's FRC material and fabrication process will be different, giving rise to variations in matrix chemistry and thickness, and fiber chemistry and fiber diameter, requiring that the ablation parameters be optimized to achieve the best results for any given FRC.

FIG. 6 illustrates two sample coupons of FRC material prior to applying the laser ablation process. The coupon on the left is a rigid carbon-fiber reinforced polymer (CFRP) panel and the coupon on the right is an electrical grade glass-fiber reinforced polymer (GFRP) sheet. These coupons are representative of any commercially-available FRC products in the market, which use any number of available commercial grade polymers as the polymer/composite matrix. The approximate dimensions of the coupons were 3 mm×25 mm×75 mm. Various settings/parameters (A, E, and F on the carbon fiber coupon and A, C, and D on the fiberglass coupons) were used during the ablation process in order to find the right settings to expose as many fibers as possible without damaging the fibers. FIG. 7 illustrates the laser ablated coupons, which were prepared using a one-micron wavelength UV (ultrafast laser), 500 fs (0.0005 ns) laser at <10 W. Based on SEM analysis, sections that were ablated using parameters C and E had the largest amount of exposed fibers. Section C on GFRP panel in FIG. 7 was ablated using a wavelength of 343 nm, pulse length of approximately 230 fs, energy of 33 micro joules, and five passes. The exact same parameters were used to ablate the sections E on CFRP substrate except that only two of passes were used to prepare this section. Thus, for these particular coupon samples, the above parameters (C and E) provided the best fiber exposure with the least fiber damage. As mentioned above, the optimal parameters will vary depending upon the FRC material being used.

The laser ablation results in a geometrically complex, high surface area region or surface to which the sprayed material can then be applied. FIG. 8 illustrates a SEM micrograph image of the laser ablated CFRP coupon of ablation parameter A. FIG. 9 illustrates a SEM micrograph image of the laser ablated CFRP coupon of ablation parameter E. FIG. 10 illustrates a SEM micrograph image of the laser ablated CFRP coupon of ablation parameter F. FIG. 11 illustrates a SEM micrograph image of the laser ablated GFRP coupon of ablation parameter A. FIG. 12 illustrates a SEM micrograph image of the laser ablated GFRP coupon of ablation parameter C. FIG. 13 illustrates a SEM micrograph image of the laser ablated GFRP coupon of ablation parameter D.

In FIGS. 8-13, notice how the fibers are exposed to the treated surface with the polymer matrix partially removed or ablated. The fiber microstructural behavior illustrates the exposure of individual fibers. Continuousness and consistency of fibers were observed in the carbon fiber samples. Based on the SEM analysis, the laser ablation technique using setting parameters E (see FIG. 9) removed the most top surface material on the carbon fiber coupon, while the laser ablation technique using setting parameters C (see FIG. 12) removed the most top surface material on the fiberglass coupon. This again confirmed that different settings can be chosen and/or optimized depending upon the FRC material to be coated.

Once the polymer matrix is removed, the coating can be applied to the treated surface of the FRC material using a thermal spray process (e.g., plasma-arc spray, suspension plasma spray (SPS), solution precursor plasma spray (SPPS), flame spray, electric-arc spray, and cold spray). With the exception of the discussion below relating to advantages and criticality of particle size and particle distribution, one skilled in the art will be otherwise able to determine the appropriate thermal spray application parameters depending upon the type of thermal spray process to be used, as well as the specific materials being coated. With the fibers exposed and the polymer matrix partially removed, preferred bonding mechanisms can be engaged to securely adhere the coating. The coating is bonded, at least in part, directly to the fibers. The two dominant bonding mechanisms of diffusion and the formation of an intermediate compound work well to bond the coating to the CFRP or GFRP. The ablated or peel ply surface is too rough for van der Waals forces to add significantly to adhesion. A typical coating would be aluminum on the exposed glass fibers, where aluminum silicate forms at the interface between the coating and the glass fiber substrate. FIGS. 14A and 14B show alumina on glass epoxy diffusion. Other coating materials could include zirconium, magnesium, beryllium, gadolinium, neodymium, and other silicate formers. Another typical coating would be either tungsten or titanium metal on the carbon fibers forming tungsten carbide at the coating/fiber interface. FIGS. 15A and 15B illustrate tungsten on carbon fiber, showing formation of tungsten carbide at the interface. Other coating materials could include titanium, tantalum, vanadium, zirconium, hafnium, chromium, or other carbide formers in the Group 4, 5, and 6 metals, and possibly others such as boron, silicon, etc. Those skilled in the art will understand that those material combinations of fibers and coatings that will more readily form carbides, silicates, and so forth are the more simple systems to understand and manipulate according to the present invention, although other reactions can be forced.

Depending on the type of fiber weave in the composite, some mechanical interlocking of the coating material is still possible—where the molten materials could get under the negative relief created by the fiber weave or bundle. However, the dominant bonding mechanisms are diffusion and the formation of an intermediate compound.

The exposed fibers generally have small diameters, e.g. 5-10 micrometers, which are subject to mechanical and/or thermal damage from the relatively large, super-heated, energetic particles in the thermal spray stream. Nominal powder distributions for plasma spray are 10-44 micrometers in diameter or larger as are particles for flame spray and EAS. To minimize thermal and mechanical damage, an atypical powder particle sizing and/or distribution is used. In one embodiment, the particle size range is reduced to 5-30 micrometers and a normal particle size distribution is used within that range. In another embodiment, a smaller or narrower particle size distribution within a larger overall particle size range is used. For example, within an overall powder particle size range of 5-100 micrometers, a narrow distribution range of 20-25 micrometers (e.g., a monomodal, Gaussian distribution) is used. For example, a distribution of particles ranging from 5-30 micrometers in size, as well as particles ranging from 80-100 micrometers in size. Such particle size and distribution selection has been found to reduce or essentially eliminate fiber damage. In some embodiments, the coating material includes SPS slurry feedstock with single digit or sub-micron particulate sizing (e.g., 9 micrometers or smaller). In another embodiment, the coating material includes SPPS metalorganic feedstock with single digit or sub-micron particulate sizing (e.g., 9 micrometers or smaller).

Various features and advantages of the invention are set forth in the following claims. 

What is claimed is:
 1. A coated fiber-reinforced composite workpiece comprising: a fiber-reinforced composite substrate including fibers distributed in a matrix; and a thermal spray coating applied to the substrate, the coating being bonded directly to the fibers via diffusion of the coating into the fibers, the formation of an intermediate compound at an interface of the coating and the fibers, or both.
 2. The coated fiber-reinforced composite workpiece of claim 1, wherein a particle size of the thermal spray coating ranges from 5-30 micrometers.
 3. The coated fiber-reinforced composite workpiece of claim 1, wherein a particle size of the thermal spray coating includes a distribution of 20-25 micrometers within a particle size range from 5-100 micrometers.
 4. The coated fiber-reinforced composite workpiece of claim 1, wherein the coating includes suspension plasma spray (SPS) having a slurry feedstock with particulate sized at 9 micrometers or smaller.
 5. The coated fiber-reinforced composite workpiece of claim 1, wherein the coating includes solution precursor plasma spray (SPPS) having a metalorganic feedstock with particulate sized at 9 micrometers or smaller.
 6. The coated fiber-reinforced composite workpiece of claim 1, wherein the workpiece is a glass-fiber reinforced polymer and the coating includes aluminum, or zirconium, or magnesium, or beryllium, or gadolinium, neodymium, or other silicate formers bonded directly to exposed glass fibers, and wherein a silicate forms at an interface between the coating and the glass fibers.
 7. The coated fiber-reinforced composite workpiece of claim 6, wherein the coating contains aluminum bonded directly to exposed glass fibers, and wherein aluminum silicate forms at an interface between the coating and the glass fibers.
 8. The coated fiber-reinforced composite workpiece of claim 1, wherein the workpiece is a carbon-fiber reinforced polymer and the coating includes tungsten, or titanium, or tantalum, or vanadium, or zirconium, or hafnium, or chromium, or other carbide formers in the Group 4, 5, and 6 metals, or boron, or silicon, and wherein a carbide forms at an interface between the coating and the carbon fibers.
 9. The coated fiber-reinforced composite workpiece of claim 8, wherein the coating contains tungsten bonded directly to exposed carbon fibers, and wherein tungsten carbide forms at an interface between the coating and the carbon fibers.
 10. The coated fiber-reinforced composite workpiece of claim 1, wherein the composite workpiece includes a glass-fiber reinforced polymer.
 11. The coated fiber-reinforced composite workpiece of claim 1, wherein the composite workpiece includes a carbon-fiber reinforced polymer.
 12. A method of coating a fiber-reinforced composite material, the method comprising: providing a fiber-reinforced composite workpiece; treating a surface of the workpiece to remove at least a portion of a polymer matrix and to expose fibers to a treated surface of the workpiece; and coating the treated surface of the workpiece using a thermal spray coating process.
 13. The method of claim 12, wherein the fiber-reinforced composite material is a carbon-fiber reinforced polymer.
 14. The method of claim 13, wherein the coating contains tungsten, or titanium, or tantalum, or other carbide formers in the Group 4, 5, and 6 metals, or boron, or silicon.
 15. The method of claim 12, wherein the fiber-reinforced composite material is a glass-fiber reinforced polymer.
 16. The method of claim 15, wherein the coating contains aluminum, or zirconium, or magnesium, or beryllium, or gadolinium, or neodymium, or other silicate formers.
 17. The method of claim 12, wherein treating a surface includes laser ablating the surface.
 18. The method of claim 17, wherein the laser ablating is performed using a UV laser.
 19. The method of claim 12, wherein the coating includes a material having a melting point greater than 1,000 C.
 20. The method of claim 12, wherein the thermal spray coating process is one of plasma-arc spray, suspension plasma spray (SPS), solution precursor plasma spray (SPPS), flame spray, electric-arc spray, or cold spray.
 21. The method of claim 12, wherein treating a surface includes a peel ply process followed by at least one of polishing, sanding, or grit blasting.
 22. The method of claim 12, wherein coating the treated surface includes coating with a thermal spray coating having a particle size distribution of 20-25 micrometers within a particle size range from 5-100 micrometers.
 23. The method of claim 12, wherein coating the treated surface includes coating with suspension plasma spray (SPS) having a slurry feedstock with particulate sized at 9 micrometers or smaller.
 24. The method of claim 12, wherein coating the treated surface includes coating with solution precursor plasma spray (SPPS) having a metalorganic feedstock with particulate sized at 9 micrometers or smaller. 